It is generally known that the printed copies produced by rotary printing presses, in particular offset rotary printing presses, present a printed impression that depends on the structure of the material to be printed, the structure or composition of the printing ink, and the layer thickness on the material to be printed, the ink areas or dots on the material to be printed, the type of light source illuminating the printing, and on the surroundings of the printed area observed
In the prior art, process problems, and in particular deviations, occurred between the command values and actual values (sensed in the printer) for the ink. These prior art systems included automatic regulating systems which relied solely on zonal variation of the ink metering, or variation of the ink ductor parameters for control. For instance, by measuring the diffuse reflections, ink layer thickness changes can be ascertained and can be corrected by changing the ink metering. However, merely changing the ink metering, in turn changes the balance between the ink and the damping liquid. Changing the ink and damping liquid balance, in turn, causes an additional change in the diffuse reflections. If only the ink metering is changed, this causes a change in the extent of contrast in the printed image. Accordingly it would be better to make an additional correction in the damping liquid metering in order to restore the previously existing balance between the ink and damping liquid. Problems that can be ascertained by measurement of diffuse reflection are particularly critical and cannot be eliminated by either changing the ink metering or by changing the damping liquid metering; examples of these are slipping and blurring.
In the prior art detecting diffuse reflection values after printing had significant problems. The prior art cannot evaluate the various behaviors of the diffuse reflections of solidly printed fields and half-tone fields, and half-tone areas of varying area coverage nor can the prior art calculate current regulating recommendations or provide diagnosis data, because other factors such as the actual printing history (or, in other words, the course of development of the measurement data) was not known accurately enough, and could not be currently evaluated.